1. Field of the Invention
The present invention relates to a wiring substrate for a ball grid array (BGA) package and a method for manufacturing such a wiring substrate and in particular, to a wiring substrate having a non-solder mask defined (NSMD) type solder ball pad for a BGA package and a method for manufacturing such a wiring substrate.
2. Description of the Related Art
The electronic industry desires to manufacture products with light weight, small size, high speed, multi-function, high performance and high reliability at competitive prices. The package assembly technique achieves this goal. With the package assembly technique, a ball grid array (BGA) package has been developed. The BGA package has advantages to a conventional plastic package, for example reduced mounting area and improved electrical characteristics.
The BGA package uses a wiring substrate, such as a printed circuit board (PCB), instead of the lead frame used by the conventional plastic package. The wiring substrate provides an entire surface for forming solder balls, allowing improved mounting density on a motherboard.
Referring to FIGS. 1 through 4, a wiring substrate 10 for a BGA package has wiring patterns 30. The wiring patterns 30 include solder ball pads 32 for forming solder balls 50 as external connection terminals. The wiring patterns 30 formed on the lower surface of the wiring substrate 10 are electrically connected to the wiring patterns (not shown) formed on the upper surface of the wiring substrate 10 through via holes 38. The solder ball pads 32 may be non-solder mask defined (NSMD) type solder ball pads. The solder ball pad 32 and a portion of the connecting pattern 34 connected to the solder ball pad 32 are exposed through an opening 43 of a solder mask 40. Reference numeral 24 is an exposed portion of a substrate body 20.
The wiring pattern 30 is formed on the lower surface of the substrate body 20 and may comprise a copper wiring layer. The solder ball pad 32 and a portion of the connecting pattern 34 are exposed through the opening 43 of the solder mask 40 and may be liable to oxidation. After formation of the opening 43, a plating layer 36 is formed to protect the exposed portion of solder ball pad 32 and connecting pattern 34. The plating layer 36 may also establish a reliable connection with the solder ball 50. The plating layer 36 may typically comprise a nickel plating layer 35 and a gold plating layer 37. Before formation of the plating layer 36, an acid cleaning process may be performed for removal of an oxidation film formed on surfaces of the solder ball pad 32 and the connecting pattern 34.
The conventional NSMD type substrate 10 has defects. For example, pattern cracks may occur during a reliability test, such as a thermal cycling test following the manufacture of the BGA package. The pattern crack relates to where a portion of connecting pattern 34 adjacent to a boundary surface 43a is cut off from the boundary surface. The pattern cracks may result from stresses concentrated on the boundary surface 43a located between the opening 43 and the connecting pattern 34, due to the difference of coefficients of thermal expansion (CTE) between the solder mask 40 and the copper wiring layer materials being great. The CTE of a typical solder mask 40 is 60×10−6/C. ° or 160×10−6/C. ° depending on the material, while that of a typical substrate body 20 is 16×10−6/C. ° and typically a copper wiring layer has a CTE of 16×10−6/C. °.
Further, the exposed solder ball pad 32 and connecting pattern 34 may be etched during the acid cleaning process. The exposed connecting pattern 34 may thereby be thinner than the connecting pattern 34 covered by the solder mask 40. The difference of thickness may generate a step 33 at the boundary surface 43a. 
As described above, the plating layer 36 is formed on the copper wiring layer including the solder ball pad 32 and the connecting pattern 34. The boundary surface 43a may form a boundary between a copper/nickel/gold structure and a copper/solder mask structure.
Accordingly, stresses may concentrate on the boundary surface 43a during thermal cycling. The pattern cracks may occur at the connecting pattern 34 located adjacent to a relatively weak portion, i.e. the boundary surface 43a. 